Radioactive leak at Fukushima

The severity warning about a toxic water leak at the Fukushima nuclear plant is to be dramatically raised, its nuclear watchdog said on Wednesday.

It is the most serious action since the plant was destroyed by an earthquake and tsunami in 2011.

The deepening crisis at the Fukushima plant will be upgraded from a level 1 "anomaly" to a level three "serious incident" on an international scale for radiological releases, a spokesman for Japan's Nuclear Regulation Authority said.

Contaminated water escapes from the Fukushima nuclear power facility. An estimated 300 tonnes of radioactive water is believed to have leaked out. Photo: AFP

That will mark the first time Japan has issued a warning on the International Nuclear Event Scale since three reactor meltdowns after the massive quake in March 2011.

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Water still leaking from the plant is so contaminated that a person standing close to it for an hour would receive five times the annual recommended limit for nuclear workers in a year.

A maximum level 7 was declared at the battered plant after explosions led to a loss of power and cooling two years ago, confirming Fukushima as the worst nuclear accident since Chernobyl a quarter of a century earlier.

Workers stand on storage tanks at the Fukushima nuclear plant in Japan on Tuesday. A warning is being upgraded over toxic water emitting from the plant. Photo: AP

The contaminated water is leaking from a storage tank, the plant's operator said on Tuesday. The leak was classified as an "anomaly" earlier this week.

The regulation authority's impending assessment upgrade came in a document posted on the agency's website on Wednesday, with formal adoption to follow a meeting that is being held by the authority's commissioners, the NRA spokesman said by telephone.

"Judging from the amount and the density of the radiation in the contaminated water that leaked ... a level 3 assessment is appropriate," the document said.

Local government officials and nuclear experts inspect attempts to prevent the seepage of contaminated water into the sea earlier this month. Photo: AFP

The leak has not been plugged.

Each one-step INES increase represents a tenfold increase in severity, according to a factsheet on the website of the International Atomic Energy Agency.